1. Field of the Invention
Embodiments of the present invention relate to an overcurrent detection apparatus that protects an insulated gate bipolar transistor (“IGBT”) from breakage when an overcurrent flows through the IGBT, and also relates an intelligent power module using the same.
2. Discussion of the Background
A three-phase inverter circuit constituting a power conversion apparatus is configured by connecting series circuits, in which two each of six IGBTs and six freewheeling diodes (“FWD”) connected to the IGBTs in anti-parallel are connected in series, in parallel to a direct current power supply, and an inductance load such as an electric motor is connected to connection points between the IGBTs of the respective series circuits.
The IGBT used in this type of power conversion apparatus is provided with an overcurrent protection circuit that protects the IGBT from breakage when an overcurrent flows through the IGBT.
FIG. 5 shows a configuration of such a conventional overcurrent protection circuit. An IGBT 100 is provided with a collector 100c, an emitter 100e, and a sense emitter 100se. The sense emitter 100se outputs a sense current that is approximately several thousand times or several tens of thousands of times smaller than a current flowing between the collector 100c and the emitter 100e. In the overcurrent protection circuit, a current detection resistor 101 is interposed between the sense emitter 100se and the ground, and a high potential side sense voltage Vse of the current detection resistor 101 is input into a non-inverting input terminal of an overcurrent detection comparator 102.
A threshold voltage Vth for determining an overcurrent is input into an inverting input terminal of the overcurrent detection comparator 102, and when the sense voltage Vse is equal to or larger than the threshold voltage Vth, an overcurrent detection signal Soc inverted from an OFF condition to an ON condition is output from the overcurrent detection comparator 102.
The overcurrent detection signal Soc is supplied to a low pass filter circuit 103 and integrated at a predetermined time constant in order to prevent erroneous detection of an overcurrent in a transient increasing condition in which the sense voltage Vse generated when the IGBT 100 is turned ON takes a value exceeding the threshold voltage Vth.
Note that the IGBT 100 is drive-controlled by a gate current supplied from a gate drive circuit 105. The gate drive circuit 105 includes a P-channel MOSFET 108 and an N-channel MOSFET 109 that are connected in series between a control power supply 106 and a ground 107, and a connection point between the P-channel MOSFET 108 and the N-channel MOSFET 109 is connected to a gate 100g of the IGBT 100. The P-channel MOSFET 108 and the N-channel MOSFET 109 are controlled by a driver IC 110 having a protective function such that when one thereof is in an ON condition, the other is in an OFF condition.
An operation of the overcurrent protection circuit described above will now be described using a signal waveform diagram shown in FIGS. 6A, 68, 6C, and 6D.
When the P-channel MOSFET 108 of the IGBT drive circuit is switched ON, a control voltage Vgcc that is equal to a voltage Vcc (approximately 15 V, for example) of the control power supply 106 is applied to the gate 100g of the IGBT 100.
The IGBT 100 is an insulated gate type semiconductor device, or a so-called voltage driven device, and a gate current Ig flows through the gate 100g of the IGBT 100 in order to charge a gate capacitance (here indicating a gate-emitter capacitance). When the gate capacitance is charged, a gate voltage Vg rises, as shown in FIG. 6B. When the gate voltage Vg rises so as to reach a gate threshold voltage, a collector current Ic rises, as shown in FIG. 6A, whereupon a collector-emitter voltage Vce starts to decrease, as shown in FIG. 6A.
Further, a sense current Ise that is approximately several thousand times to several tens of thousands of times smaller than the collector current Ic rises, as shown in FIG. 6 and voltages at respective ends of a sense resistor Rs through which the sense current Ise flows, or in other words a sense voltage Vse, also increases, as shown in FIG. 6D. When the gate voltage Vg reaches the gate threshold voltage, the collector-emitter voltage Vce decreases, leading to an increase in a mirror capacitance (a gate-collector capacitance) of the IGBT 100, and as a result, the gate voltage Vg shifts to a substantially constant region.
Meanwhile, the sense current Ise also increases, as shown in FIG. 6C, leading to an increase in the sense voltage Vse, as shown in FIG. 6D, and when the sense voltage Vse reaches an overcurrent threshold voltage Vth (which is determined by a reference voltage E) at which an overcurrent is determined, an output of the overcurrent detection comparator 102 outputs an H level overcurrent detection signal Soc.
The overcurrent detection signal Soc is supplied to the low pass filter circuit 103, and therefore an output of the low pass filter circuit 103 increases gently. At this time, the time constant of the low pass filter circuit 103 is set to be larger than a misdetection prevention period T1 extending from a point at which the sense voltage Vse exceeds the overcurrent threshold voltage Vth to a point at which the sense voltage Vse falls to or below the overcurrent threshold voltage Vth.
Hence, during the misdetection prevention period T1 in which the sense voltage Vse exceeds the overcurrent threshold voltage Vth, the filter output of the low pass filter circuit 103 does not reach the H level of the overcurrent detection comparator 102.
When the filter output of the low pass filter circuit 103 is supplied to the driver IC having a protective function, therefore, the filter output does not reach the H level of the overcurrent detection comparator 102. As a result, the gate current Ig is not blocked by the driver IC 110 having a protective function.
However, when the H level of the overcurrent detection signal Soc output from the overcurrent detection comparator 102 is maintained beyond the misdetection prevention period T1, the filter output of the low pass filter circuit 103 reaches the H level. As a result, output of the gate current Ig to the gate 100g of the IGBT 100 is stopped by the driver IC 110 having a protective function, whereby the overcurrent protection function of the IGBT 100 is activated.
In the above configuration shown in FIG. 5, a case in which erroneous overcurrent detection when the IGBT 100 is turned ON is prevented using the low pass filter circuit 103 was described, but a configuration such as that described in Japanese Patent Application Publication No. H6-120787 (“Patent Document 1”)may be employed instead.
In Patent Document 1, the low pass filter circuit is omitted, and a timer is activated upon detection of a rising edge of an input signal for activating the IGBT. As a result, a higher reference voltage than a normal reference voltage is supplied to the overcurrent detection comparator during the aforesaid misdetection prevention period T1, whereby the overcurrent detection signal that is output from the overcurrent detection comparator within the period in which the sense voltage Vse exceeds the overcurrent threshold voltage Vth is maintained at an L level.
Incidentally, according to the conventional examples described above, either the rise of the H level overcurrent detection signal Soc output from the overcurrent detection comparator 102 is smoothed using the low pass filter circuit, or a higher reference voltage than the normal reference voltage is applied to the overcurrent detection comparator during the misdetection prevention period T1 using the timer. In so doing, an erroneous detection is prevented from occurring during the period in which the sense voltage Vse generated when the IGBT is turned ON exceeds the overcurrent threshold voltage Vth.
In the conventional examples described above, however, when the low pass filter circuit is used, the misdetection prevention period T1 of the IGBT is, at 4 to 5 μsec, comparatively long, and the overcurrent determination is performed after the misdetection prevention period T1 has elapsed. Hence, a problem remains unsolved in that when an overcurrent condition occurs, a determination time required to determine the overcurrent condition lengthens. Moreover, the misdetection prevention period T1 differs according to the IGBT, and therefore the time constant of the low pass filter circuit must be set to be relatively long. Likewise in this respect, a problem remains unsolved in that the determination time required to determine an overcurrent condition lengthens. As a result, simultaneous detection of a short circuit current, which requires that the determination be performed quickly, is impossible.
Meanwhile, when the reference voltage of the overcurrent detection comparator is modified using the timer, as in the invention described in Patent Document 1, a low pass filter is not used, and therefore the need to set a time constant is eliminated. In the invention described in Patent Document 1, however, it is necessary to set a comparatively long time up period corresponding to the misdetection prevention period T1 on the timer. Since the misdetection prevention period T1 differs according to the IGBT, a problem remains unsolved in that the time up period must be set to be relatively long. Furthermore, in the invention described in Patent Document 1, the timer, a switch circuit, two types of reference voltage sources, and so on must be provided, and therefore a problem remains unsolved in that the circuit configuration increases in size.